Precious transition metal irons and their coordination complexes find industrial applications as supported catalysts and performance chemicals in fine chemicals industries. For example, metals like Ag, Au, Pd, Pt and Rh are used in a variety of industrial applications as catalysts for oxidation, hydrogenation and dehydrogenation reactions. Various polymeric materials modified silica, zeolite or/and various clay materials are used as support for these metals. Similarly, coordination metal complexes of Pd, Pt and Rh are used as commercial catalysts in homogeneous conditions for hydroformylation and hydrogenation reaction. However, owing to difficulties in separation of metal complexes from product mixtures, there is growing need to developed heterogenized catalysts where coordination metal complexes are supported on polymeric or inorganic solid support like silica, carbon, zeolite and alumina. Commercially, it is important to recover the precious metals from the support to the maximum extent possible once the catalyst is deactivated.
In certain situations, the catalysts are degraded during the reaction cycle or subsequent work-up and the reaction effluent may comprise of various remnants, e.g., complexes of various valence states for the metal ions. In many cases, the presence of excessive contaminants reduces the feasibility of recycling of the catalyst. However, the problem related to degradation of catalysts over the repeated cycles, contaminants over the repeated cycles, contaminants arise from side reaction and leaching of the effective catalyst for the solid support are common in some cases of heterogeneous catalysis. In view of the environmentally stipulated restrictions on disposal of metal containing waste, effective recovery of the residual precious metals from the reaction effluent is of paramount importance for a process to be environmentally acceptable and economically viable.
Furthermore, during the recovery of precious metals from ore or scrap including spent catalysts, the use of solvent extraction to separate the precious metals from one another and from base metals that may also be present is becoming more widespread. The hydrometallurgical processes employed for the separation and recovery of the platinum group metals, (e.g., platinum, palladium and rhodium), typically involve dissolving the metal ions by some type of oxidative acidic chloride leach, typically with aqua regia or hydrochloric acid/Cl2 followed by solvent extraction.
The application of coordination complexes of precious metals in fine chemicals industries also requires prior purification of these complexes using silica gel as a stationary phase. For example Pd-pythalocyanine complexes namely Palladium phthalocyanine complex, Bromo phthalocyanine complex and Irgaphor green, which find applications as pigment material for compact disc coating are purified using silica gel.
Significant quantity of these complexes (up to 0.5 wt % Pd) is adsorbed in the pores of silica gel and is difficult to dislodge from the pores of silica (spent silica) by conventional elution with a solvent. Many technologies, at times specific to support and metal are used for recovery of precious metals from spent catalysts, effluent solutions.
Reference is made to the patent (Great Britain Patent no. A-2127001, 1984) wherein precious metals are recovered rapidly and efficiently from cyanide containing leach solution by loading onto an activated carbon fibre body. The main limitation of the process is that cyanide ligand should have high affinity and should form stable complexes with the metal ions to be leached out.
Reference is made to the patent CS-B-251467 (1988); Chemical Abstract 109 (24): 213964) wherein palladium is recovered from acidified wastewater by sorption on activated carbon, pre-treated with an alkali metal salt of Ethylene diamine tetra acetic acid (EDTA). However, this is useful for homogenous aqueous catalyst waste and is not directly applicable to non-aqueous systems.
Reference is made to the patent JP 54 9597 (1978) wherein, the regeneration and recovery of precious metal like palladium involves a burning step. It is disclosed in that patent, that the spent residue obtained from a catalyzed reaction performed in homogeneous condition is subjected to heating to burn out the organic fragment and then by subsequent dissolution of the palladium species in acid to the corresponding palladium salt. However, this invention has a limited scope in the sense that it is only concerned with degradation of metal complex having lower aliphatic mono-carboxylic acids as the organic moiety.
Reference is made to the U.S. Pat. No. 4,331,634 (1982) wherein strongly acidic solution of sulfuric, hydrochloric, perchloric or nitric acid is used as stripping solvent for the extraction of the palladium from the organic acidic solution containing oxime as extracting reagent. The organic phase may also contain an anionic phase transfer material or catalyst to aid the extraction process. In this patent, the oximes employed for the extraction of palladium are hydroxyoxime and derivatives, which are expensive considering the industrial application.
Reference is also made to the U.S. Pat. No. 4,578,250 (1986) wherein aqueous ammonia is used as the stripping solution and along with oxime as an extracting solvent similar to those of the U.S. Pat. No. 4,331,634 (1982) but here oxime used is ether oxime, in which, the hydroxy group of the oximes are converted to ether groups. In this, case also, the oximes employed for the extraction of palladium is expensive-and not user friendly considering the industrial application. Furthermore, it is reported that after recycling certain oxime containing solvents over a period of time (e.g., ten extractions of palladium followed each time by stripping with 6M hydrochloric acid solution), the rate of palladium extraction deteriorates considerably.
Reference is made to the JP Patent 61-238, 927 (1996) wherein palladium is recovered by extraction using an aldoxime. The main limitation of these processes is the use of expensive organic ligands and organic solvents, which are difficult to recover.
Y. Baba, K. Inoue, K. Yoshizuka and T Furusawa, (Industrial engineering Chemistry—Research, Volume 29, (1990), page 21111) have described the use of non-chelating oximes such as dodecanal oxime, decanal, oxime, octanal oxime and hexanol oxime for the extraction of palladium metal. This report also has drawback in the sense that the oximes employed for the extraction are expensive considering the industrial application.
Ion exchange resins can also be used to recover precious metal ions, but it is difficult to achieve 100% metal recovery in this method and all precious metals are not also not present in the ionic state or too much of contaminants is present in the effluents to be treated.
According to the above prior art, most of the metal recovery methods are applicable to the homogeneous reaction mixtures and employs chelating agents for extracting metal ions. Process known in the prior art for recovery of metals from supported catalysts also makes use of mineral acids and invariably destroyed the support structure.